Electronic musical instruments of the synthesizer type use a variety of spectral transformations to produce special effects. One effect is that of the resonator. For example, a tone synthesizer may employ a low-pass or high-pass filter in which the cutoff frequency is made to vary as a function of time. The resonator effect is usually produced by a tuned resonant circuit that follows or precedes the low-pass filter, the resonator operating to enhance harmonic overtones near the cutoff frequency of the filter. The tonal effect of the resonator is referred to as the Q-accent effect.
The Q-accent effect has been implemented in digital tone generators of the type in which the musical wave shapes are synthesized by a Fourier calculation using stored harmonic coefficients. For example, in U.S. Pat. No. 3,956,960, entitled "Formant Filtering in a Computer Organ", there is disclosed means for eliminating, attenuating or accentuating certain Fourier components included in each wave shape amplitude computation. A set of formant filter factors define the formant filter pass band as a function of frequency, logarithmic frequency, or Fourier component order. As each Fourier component is independently evaluated, the amplitude is scaled by the appropriate formant filter factor. The resulting amplitude values produce a synthesized musical tone which includes only Fourier components within the defined pass band, so that formant filtering effectively is implemented without the use of an actual filter. An alternative arrangement is disclosed in U.S. Pat. No. 4,000,675, entitled "Electronic Musical Instrument", in which the harmonic attenuation is accomplished by a calculation algorithm, rather than a stored set of scale factors. Neither of these arrangements, however, is suitable for generating a Q-accent effect in an analog synthesizer or in a digital organ of the type described in U.S. Pat. No. 3,515,792, for example, where the harmonic components are not separately calculated or stored.